1. Motivation: Identify the Problem

Insects, particularly honey bees, play a crucial role in sustaining both natural ecosystems and human society. They are key pollinators, ensuring the reproduction of flowering plants and contributing significantly to food production. Given their essential role, the need for precise and continuous monitoring in dynamic environments is undeniable. Reliable insect monitoring is critical for understanding the changing conditions of these habitats.

To protect and monitor these crucial insects, innovative, non-invasive, and continuous monitoring systems are required to assess their interactions with their environment. Traditional methods are labor-intensive and often inaccurate, which makes wireless communication technologies, like terahertz (THz) sensing, a promising alternative. The THz region, with its non-ionizing and non-invasive properties, offers a unique opportunity to explore insect monitoring without disrupting their natural environment.

For effective monitoring, several key aspects must be addressed, such as dosimetry, radar cross-section (RCS) of insects for clear identification, and the spectral characteristics that distinguish different insect species. Specifically, the correlation between bee orientation and THz wave polarization is essential for detailed analysis. Moreover, the dielectric properties of the bee’s body must be understood in a detailed and heterogeneous manner, as bees are multicomponent entities with varying electrical properties across different body regions. Our work focuses on this comprehensive investigation, providing the necessary material characterization and insights to enable accurate simulations and real-time monitoring using THz technologies.

2. Background: Challenges and Compilation of Authors

As continuous insect monitoring presents complex challenges, it was deemed essential to prepare a manuscript that highlights the vision of THz-based concepts, outlines various perspectives, and emphasizes the multi-step process involved. To achieve a comprehensive study, both measurements and simulations were incorporated. However, both approaches posed significant challenges.

Measurements on live honey bees raise ethical concerns and present two major technical issues. First, the reproducibility of measurement results is difficult, as live bees do not remain stationary during the measurement period, and their trajectory changes continuously. Second, non-living bees can serve as an approximation but only partially retain the dielectric properties of living specimens. Additionally, tissue dehydration affects their dielectric response, making it necessary to conduct measurements on the same day of specimen preparation.

To address these limitations, 3D-printed honey bees were introduced as a promising alternative. These models, made from epoxy resin and polyamide 12 (PA12), offer structural stability and remain unaffected by water loss. The 3D models were fabricated based on a photorealistic digital twin, which was subsequently employed in full-wave electromagnetic (EM) simulations. This approach enabled the prediction of THz-wave interactions in different body regions across various distances from the transceiver, as well as in near-field and far-field regions.

Since biological organisms are heterogeneous entities with spatially varying dielectric properties, it was crucial to determine the complex permittivity over the entire frequency range from 1 to 500 GHz. This was achieved using preserved honey bees obtained from beekeepers and carefully dissected into key components: wings, inner parts, cuticle, and the full bee. The frequency-dependent dielectric properties of these samples were then measured.

To validate the feasibility of 3D-printed bee models for reproducible THz measurements, synthetic aperture radar (SAR) imaging experiments were conducted. Specifically, frequency-dependent back-projection reconstructions of real and 3D-printed bees were compared using a back-projection algorithm. Additionally, rotational measurements with 90° steps were performed to investigate the correlation between bee orientation and THz wave polarization.

The attractiveness of THz technology for insect monitoring arises from its unique advantages. THz waves provide sub-millimeter spatial resolution, enabling precise characterization of small biological structures. Moreover, the ability to miniaturize hardware and antenna structures makes THz systems a viable candidate for compact and portable sensing solutions. Unlike conventional imaging techniques, THz waves can penetrate organic tissues while remaining non-ionizing and non-invasive, making them ideal for studying insect morphology and behavior without altering their natural state.

Given the diverse challenges and research directions involved, a multidisciplinary team of 24 authors collaborated to contribute their expertise in performing the necessary measurements and simulations, ensuring a holistic approach to this complex problem.  It should be noted that our approach to the European honey bee  (Apis mellifera) is an ongoing multi-staged process.

3. Results and Findings

In this study, we explored multiple measurement and simulation approaches to assess the interaction between European honey bees and electromagnetic (EM) waves in the terahertz (THz) spectrum. Realistic models of honey bees were meticulously constructed using advanced modeling techniques in Blender software, ensuring high accuracy in their representation. These models were then additively manufactured using two materials, epoxy resin and PA12, which provided the necessary structural stability and dielectric properties. The 3D-printed bee models played a pivotal role in all measurement campaigns, offering a reproducible and controlled alternative to live specimens.

Through extensive measurement and simulation approaches, we demonstrated variations in THz responses depending on different bee models, orientations, and polarization states. Our findings indicate that a minimum bandwidth of 100 GHz is necessary to resolve a bee’s contours, while a broader bandwidth of 450 GHz enhances image sharpness. Furthermore, THz time-domain spectroscopy (THz-TDS) analyses confirmed the repeatability of measurements, enabling the detection of RCS fingerprints and supporting high-resolution  inverse synthetic aperture radar (ISAR) imaging.

Our investigation revealed notable changes in the THz response during scattered propagation measurements using various bee-mimicking samples, with an resonant tunneling diode (RTD)  serving as the THz source. A key finding was the correlation between bee orientation and THz wave polarization. Frequency shifts and amplitude changes were observed, depending on the specific bee model used. These results demonstrate the potential of THz sensing to capture detailed information about insect morphology, allowing for the differentiation of various bee types and sizes. Such capabilities are essential for the continuous monitoring of insect populations, particularly in dynamic environments like beehives.

A crucial revelation of this study is the heterogeneous nature of honey bees. Detailed material characterization proved essential for predicting EM-induced stress within the THz region, improving the accuracy of full-wave EM simulations. Simulations at 300 GHz estimated that a honeybee in close proximity (1 mm) to an antenna could absorb up to 26% of the transmitted power, with the exoskeleton being the primary absorber in the near field. In the far field, the inner tissues contributed slightly more to power absorption. Importantly, compliance with International Commission on Non-Ionizing Radiation Protection (ICNIRP) reference levels was ensured, with a maximum allowable far-field exposure of 10 W/m² corresponding to an absorbed power of 0.29 mW per bee.

These findings not only underline the potential of THz technology for insect monitoring but also emphasize the necessity for continued research in this field. The unparalleled spatial resolution of THz systems could outperform sub-THz and  millimeter wave (mmWave) technologies, facilitating contactless, high-precision tracking and identification of small insects. While 3D-printed bee models provide an approximation of real dielectric properties, they remain a valuable tool for repeatable and controlled experimental investigations.

4. Outlook for Future Research

Undoubtedly, the work presented here paves the way towards a deeper understanding of honey bee spectral fingerprints derived from both simulations and measurements. Monitoring a key pollinator, specifically the European honey bee, represents significant progress towards continuous insect monitoring. However, future research must expand to encompass various other aspects, such as multi-insect monitoring, insect-insect, insect-plant, and insect-human interactions, particularly for pollinators and pests.

Future studies should also focus on the broader ecological context, especially in light of the consequences of climate change. The ability to monitor complex biotopes in dynamically changing environments could provide valuable insights into ecological shifts and biodiversity dynamics. In particular, tracking pollinators and pests is crucial for agricultural monitoring and pest management, as well as for halting the spread of tropical diseases, which increasingly threaten global ecosystems.

The next logical step involves conducting extensive field tests to validate the performance and reliability of THz technologies in real-world settings. Digital twins, which closely mimic typical biotopes, could play an essential role in simulating various environmental conditions and improving the robustness of monitoring systems. By leveraging these advanced simulations, it will be possible to optimize THz sensing techniques for diverse environmental scenarios and a wide range of insect species.

As research continues, the focus should also include expanding the range of insect species studied, particularly those involved in agricultural and ecological functions. Additionally, miniaturizing THz sensing systems for portable, field-ready applications will enhance the ability to implement large-scale, non-invasive monitoring networks capable of real-time data collection. Of course, technical aspects such as the limited transmit power of THz sources, integration challenges of smaller THz antennas, as well as beamforming and beamsteering techniques must also be considered when developing prototypes for this specific application.

By addressing these key areas, THz technologies hold the potential to revolutionize our ability to track and understand insect populations, offering novel approaches to ecological monitoring, conservation, and pest control.